Incorrect "Last modified" date for posts other than publish or future status

[cadic]

In the list table, the Date column displays the post date with the prefix:

• Published for published posts
• Scheduled for future posts
• Last modified for all other post statuses

In fact, the post creation date is still displayed after the Last Modified label.

To reproduce:

1. Open list of posts
2. Pick a post which was published a while ago, remember the date displayed
3. Change the post status (Draft or Pending Review)
4. In the list of posts, the date remains the same but the label say "Last Modified"

Expected behaviour:

• The last modification date (based on post_modified field) is displayed

A significant consequence: when posts are ordered by Date column, the post_date field is used to order. If we display post_modified field content in the column, it might be confusing.